Gulf Coast Vulnerable to Extreme Erosion During Category 1 Hurricanes—New Model to Help Community Planners, Emergency Managers
Sept. / Oct. 2012
About 70 percent of the Gulf of Mexico shoreline is vulnerable to extreme erosion and overwash during even the weakest hurricanes, according to a U.S. Geological Survey (USGS) report released just before the start of the 2012 hurricane season, which officially began on June 1 and ends on November 30. The report’s findings were underscored in late August by Hurricane Isaac, a category 1 hurricane that caused widespread erosion and overwash along the Louisiana, Mississippi, and Alabama barrier islands.
The report, titled “National Assessment of Hurricane-Induced Coastal Erosion Hazards: Gulf of Mexico,” focuses on the sandy beaches on the mainland and barrier islands of the U.S. Gulf of Mexico shoreline. These beaches are among the most vulnerable in the nation because of low coastal elevations and frequent hurricanes. The new publication presents the probabilities of dune erosion, overwash, and inundation of these beaches during direct hurricane landfall, as determined by USGS scientists using state-of-the-art modeling.
The research is expected to help emergency managers at local, state, and federal levels as they prepare for hurricanes in this and future seasons. Planners will be able to determine how different categories of hurricanes would impact their beaches and surrounding communities, helping them better protect lives and property. The report also includes an interactive map that allows users to focus on different parts of the Gulf Coast shoreline to view how the probability of erosion caused by waves and storm surge varies with hurricane intensity.
“The Gulf Coast’s beaches provide abundant recreational opportunities, contribute substantially to the local economy, and demand the highest real-estate values,” said USGS Director Marcia McNutt. “This important research raises awareness on the specific nature of the vulnerability of these beautiful beaches to impacts from even Cat-1 hurricanes so that property damage can be minimized through proper planning.”
In a storm, high waves and storm surge can act together to erode beaches and inundate low-lying lands; during hurricane landfall, these changes can sometimes be catastrophic.
“Beaches along the Gulf of Mexico are extremely vulnerable to erosion during hurricanes, in part because of low elevations along the coast,” said Hilary Stockdon, a USGS research oceanographer and lead author of the study. “For example, the average elevation of sand dunes on the west coast of Florida is 8 feet; on Florida’s Atlantic coast, the average is 15 feet.”
During the landfall of a category 1 storm, in which sustained winds are between 75 and 94 miles per hour, overwash is very likely for 70 percent of Gulf Coast beaches. Overwash, which occurs when waves and storm surge overtop dunes and transport sand landward, is likely at these sites because of increased water levels at the shoreline. During category 1 hurricanes on the Gulf Coast, wave height and storm surge combine to increase water levels at the shoreline by 14½ feet above their normal levels.
“People continue to build communities in coastal areas that shift and move with each passing storm,” said Stockdon. “This model helps us predict the potential impact of future storms and allows us to identify where the most vulnerable areas are located along the coast.”
Additional findings from the report show that during a category 1 storm landfall, 27 percent of sandy beaches along the U.S. Gulf of Mexico are likely to be inundated, which occurs when increased water levels completely submerge beaches and dunes. If category 5 storms occur, in which wind speeds are 157 miles per hour or higher, 89 percent of these beaches are likely to be inundated during a direct landfall.
USGS scientists used methodology developed from a decade of USGS research on storm-driven coastal-change hazards as the basis for these calculations. Observational data were combined with sophisticated hydrodynamic modeling to predict the coastal changes provided in the report. As new data and storm predictions become available, the report’s analysis will be updated to describe how coastal vulnerability may change in the future.
The full citation for the new report is:
Stockdon, H.R., Doran, K.J., Thompson, D.M., Sopkin, K.L., Plant, N.G., and Sallenger, A.H., 2012, National assessment of hurricane-induced coastal erosion hazards—Gulf of Mexico: U.S. Geological Survey Open-File Report 2012–1084, 51 p. [http://pubs.usgs.gov/of/2012/1084/].
Key findings from the report are posted at http://coastal.er.usgs.gov/hurricanes/erosionhazards/gom/.
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